I know it wasn't Otomo. I think the cop said something about one of the family's making it look like it was Otomo. They overtly leave a card saying the delivery is from Otomo.
I'm not sure if the cop mentions which family though and I don't rate the film enough to go back and find out.
I just assumed Kimura was killed so his turf could be reclaimed.
The film never brings it up afterward. My guess is it was orchestrated behind the scenes by the lieutenant in the Hanabishi gang (he never did like either Kimura nor Otomo) and Special Detective Kataoka in order to bring Otomo out. There was no need for either once the Hanabishi took over Sanno's territory and made a pact.
shareThanks to "Company_Scum" in this thread: http://www.imdb.com/title/tt1724962/board/nest/223116950:
remnants of those loyal to kato. in a scene before, we see kataoka get out of a car and inside the car we see katos bodyguard holding on dearly to katos hat and watch. so i'm assuming kataoka gave away kimuras location.
the Hanabishi with the co ordination of the cop.
shareThe tall unsmiling cop appeared to be convinced that his smarmy partner was heavily involved, despite the latter's protests that he wasn't.🐭
shareThe man without the hat was alongside Kato the whole film, though his rank was never made clear. One thing's for sure though: the cop wanted Otomo to kill the dudes from Osaka to even out the power a little, hence he orchestrated the killing.
So now you'll have to decide yourself: was it the cop (Kataoka? I don't remember his name too well) or the bodyguard/underling/sworn-brother/executive who was loyal to Kato beyond death, and actually pulled the trigger? I guess it all comes down to perspective.
At the end though, Otomo blames the cop... which is why the film ends the way it does.